The present invention relates in general to pressure transducers, and, in particular, to a new and useful transducer using thick film resistors.
Pressure transducers utilizing flat diaphragms with strain gages to measure pressure induced deflections are well known. (See U.S Pat. No. 3,341,794 to Stedman; U.S. Pat. No. 3,456,226 to Vick and U.S. Pat. No. 3,505,634 to von Vick). In general, these transducers utilize flat metal or silicon diaphragms with thin-film, bonded foil, or silicon type strain gages on them. The strain gages are placed on the diaphragm to respond to tensile, tangential strains at the center and compressive, radial strains at the outer edge of the diaphragm.
As shown in FIGS. 1 and 2, a cylindrical ceramic diaphragm 1 carries a plurality of thick film resistors 2 positioned to respond to radial strains. Thick film resistors 3 are provided near the center of the disk for responding to tangential strains. FIGS. 3 and 4 show resistors 4 and 5 oriented on a diaphragm 6. The resistors are oriented so that the strain they are meant to measure is parallel to the resistors' longitudinal axis as shown in FIG. 4. The resulting resistors are limited in size to the size of the diaphragm. P shows the direction of applied pressure to the diaphragm.
The strain gages are connected in a Wheatstone bridge configuration so that adjacent legs of the bridge sense strains of opposite sign resulting in an additive effect to the transducer output. Recently, ceramic diaphragms with screen printed and fired thick film resistors have attracted interest as pressure transducers (see U.S. Pat. No. 4,311,980 to Prudenziati and "The Thick Film Strain Gage", Howard A. Nielsen, Jr, ISA 32nd International Instrumentation Symposium, Paper Session 4.7, May 8, 1986). These ceramic/thick film transducers have been designed in a manner very similar to the transducers before them with the resistors arranged in a Wheatstone bridge configuration.
Thick film resistors are known to be susceptible to a problem known as "burst noise" (see "Physical Model of Burst Noise in Thick Film Resistors", T. M. Chen and J. G. Cottle, Solid State Electronics, Vol. 29, No. 9, pp 865-872, 1986"; and "Characteristics, Sources and Minimization of Thick Film Resistor Burst Noise", J. G. Cottle and T. M. Chen, ISHM Proceedings of the International Symposium on Microelectronics, 1986, pp. 835-839). This phenomena can cause noise in the output signal of a ceramic/thick film pressure transducer with a magnitude greater than 0.15% of span. Burst noise has been shown to be minimized by using thick film resistors of low resistivity and large size. In pressure transducer applications, it is desirable to have high resistance to minimize power consumption. The resistance of a thick film resistor is given by: ##EQU1## where R is the resistance, p is the resistivity of the resistor compound, 1 is the length of the resistor, w is the width of the resistor, and t is the thickness of the resistor. To obtain high resistance, a high resistivity resistor compound or a large area resistor are required, that is, a long and narrow resistor is required. Since low noise and high resistance are desired, a lower resistivity thick film material must be used with a large area, that is, a long and narrow resistor.
Thick film resistors change resistance as a function of the average strain over the area of the resistor. To obtain maximum output, the resistors should be close to the center or close to the outer edge of the diaphragm to maximize the average strain level experienced by the resistor. A large diaphragm is required to accomodate four long and narrow resistors in the previously used Wheatstone bridge configuration and keep the average strains at the resistors high enough for good output. Since stresses in the diaphragm increase as a function of the square of the radius, large diaphragms have high stresses which are undesirable. There is not enough room on a small diaphragm to locate a full bridge using long and narrow resistors required for good noise performance. A half or quarter bridge has a smaller output than desired.
In the prior art, there is not room on the diaphragm for this type of resistor unless the diaphragm is very large. As diaphragm size becomes larger, the stresses in the diaphragm increase as a function of the square of the radius. This is undesirable from a diaphragm strength standpoint.
In the prior art, the output of the thick film resistor has been due to the radial and tangential strains in the ceramic diaphragm in the directions parallel and perpendicular to the gage axis. Thick film resistors to date have taken advantage of the gage factor of the resistor in a direction parallel to the resistor axis and the gage factor perpendicular to the resistor axis. The change in resistance for a thick film resistor experiencing a strain has been determined to date by: ##EQU2## where ##EQU3## is the resistance change caused by a strain parallel to the length of the resistor, ##EQU4## is the resistance change cause by a strain perpendicular to the length of the gage, GF.sub.t is the gage factor of the resistor perpendicular to the length of the gage, GF.sub.I is the gage factor of the resistor parallel to the length of the gage e.sub.x is the strain parallel to the length of the gage, and e.sub.y is the strain perpendicular to the length of the gage. See J. Phys. D; Applied Physics, Vol. 12, 1979, pp. L51-53 "Strain Sensitivity in Film and Cermet Resistors: Measured and Physical Quantities", Morten et al.; IEEE Transactions on Components, Hybrids and Manufacturing Technology, Vol. CHMT-3, No. 3, Sept. 1980, Pp. 421-423 "Strain Sensitivity in Thick Film Resistors", Canali, et al; "Strain Sensitivity of Thick Film Resistors", J. S. Shah IEEE Transactions on Compon. Hybrids and Manufacturing Technology, Vol, CHMT-3, No. 4, 1980, pp. 410-420; "Changes in Thick Film Resistor Values Due to Substrate Flexure", P. J. Holmes, Microelectronics and Reliability, Vol. 12, 1973, pp. 395; and "Strain Characteristics of Thick Film Resistors and Its Application to a Strain Sensor", Osamu Abe and Yoshiaki Taketa, IMC, 1986 Proceedings, 1986, pp. 282-285.